In dealing with the transferring of liquid metals from a furnace by means of a crucible or ladle, there are a number of potential difficulties. One common type of problem deals with handling of certain metals, particularly aluminum. Molten alumninum is very reactive chemically and therefore very corrosive to metal containers. To combat this problem metal containers or crucibles are occasionally replaced by ceramic or refractory formed or lined components. The difficulty that occurs with this type of structure is that ceramic or refractory components are brittle and easily broken upon impact. Also, some are subject to thermal shock and when they are used with metal, allowance must be provided for low thermal expansion. Accordingly, it is desirable where possible to use metal crucibles or ladles to handle, transport and meter liquid metals. In fact, one type of successful crucible for this purpose is depicted and described in U.S. Pat. No. 4,073,414. The type of structure in that reference has proved to be extremely variable and useful in overcoming many problems in the liquid metal handling field.
Further improvements are desirable particularly when working with molten metals such as aluminum. If an unalloyed grey iron is used for the crucible surface, scale will form after exposure to sufficiently high temperatures. The scale formed in air consists of a mixture of iron oxides. The important factor in scale formation is whether the scale is adherent to the base metal and thus prevents further oxidation, or whether it flakes off permitting further and continuous oxidation of the base metal. In environments subject to a high degree of oxidation this consideration is of great concern.
One solution to the oxidation problem has been to provide a crucible formed of oxidized ni-resist, that is a nickel containing iron. The finished parts of the crucible to be contacted by the molten metal such as liquid aluminum are oxidized prior to use by forming an adherent layer of oxide which is more resistant chemically than the original metal. Therefore, since molten aluminum is reactive chemically, and therefore very corrosive to metal containers, the resistance to corrosion of the ultimate crucible is greatly enhanced.
Silicon and chromium increase the scaling resistance of cast iron by forming a thin surface oxide that resists further oxidation. These elements also reduce the toughness and thermal shock resistance of cast irons. The presence of nickel serves to improve the scale resistors of most alloys containing chromium and, in addition, increases the toughness and strength at elevated temperatures.
The corrosion resistance of grey iron is improved by the addition of appreciable amounts of chromium, nickel and copper, singly or in combination. Nickel irons have been chosen for crucible construction to obtain a combination of corrosion resistance, high temperature toughness and adherent oxidation resistant scale formation.
In dealing with the highly acceptable crucibles formed of ni-resist alloy with a layer of oxide thereon, eventually the oxide could wear off and consequently expose the worn surfaces to the corrosive molten metal such as aluminum. This condition would be particularly prevalent where there is interengagement between surfaces and frictional action between those surfaces. For example, this could occur where an aperature in the crucible is repeatedly opened and closed.